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  1. Spaceplates are novel flat-optic devices that implement the optical response of a free-space volume over a smaller length, effectively “compressing space” for light propagation. Together with flat lenses such as metalenses or diffractive lenses, spaceplates have the potential to enable the miniaturization of any free-space optical system. While the fundamental and practical bounds on the performance metrics of flat lenses have been well studied in recent years, a similar understanding of the ultimate limits of spaceplates is lacking, especially regarding the issue of bandwidth, which remains as a crucial roadblock for the adoption of this platform. In this work, we derive fundamental bounds on the bandwidth of spaceplates as a function of their numerical aperture and compression ratio (ratio by which the free-space pathway is compressed). The general form of these bounds is universal and can be applied and specialized for different broad classes of space-compression devices, regardless of their particular implementation. Our findings also offer relevant insights into the physical mechanism at the origin of generic space-compression effects and may guide the design of higher performance spaceplates, opening new opportunities for ultra-compact, monolithic, planar optical systems for a variety of applications.

  2. Limited-size receiver (Rx) apertures and transmitter–Rx (Tx–Rx) misalignments could induce power loss and modal crosstalk in a mode-multiplexed free-space link. We experimentally demonstrate the mitigation of these impairments in a 400 Gbit/s four-data-channel free-space optical link. To mitigate the above degradations, our approach of singular-value-decomposition-based (SVD-based) beam orthogonalization includes (1) measuring the transmission matrixHfor the link given a limited-size aperture or misalignment; (2) performing SVD on the transmission matrix to find theU,Σ<#comment/>, andVcomplex matrices; (3) transmitting each data channel on a beam that is a combination of Laguerre–Gaussian modes with complex weights according to theVmatrix; and (4) applying theUmatrix to the channel demultiplexer at the Rx. Compared with the case of transmitting each channel on a beam using a single mode, our experimental results when transmitting multi-mode beams show that (a) with a limited-size aperture, the power loss and crosstalk could be reduced by∼<#comment/>8and∼<#comment/>23dB, respectively; and (b) with misalignment, the power loss and crosstalk could be reduced by∼<#comment/>15and∼<#comment/>40dB, respectively.

  3. We experimentally demonstrate the utilization of adaptive optics (AO) to mitigate intra-group power coupling among linearly polarized (LP) modes in a graded-index few-mode fiber (GI FMF). Generally, in this fiber, the coupling between degenerate modes inside a modal group tends to be stronger than between modes belonging to different groups. In our approach, the coupling inside theLP11group can be represented by a combination of orbital-angular-momentum (OAM) modes, such that reducing power coupling in OAM set tends to indicate the capability to reduce the coupling inside theLP11group. We employ two output OAM modesl=+1andl=−<#comment/>1as resultant linear combinations of degenerateLP11aandLP11bmodes inside theLP11group of a∼<#comment/>0.6-kmGI FMF. The power coupling is mitigated by shaping the amplitude and phase of the distorted OAM modes. Each OAM mode carries an independent 20-, 40-, or 100-Gbit/s quadrature-phase-shift-keying data stream. We measure the transmission matrix (TM) in the OAM basis withinLP11group, which is a subset of the full LP TMmore »of the FMF-based system. An inverse TM is subsequently implemented before the receiver by a spatial light modulator to mitigate the intra-modal-group power coupling. With AO mitigation, the experimental results forl=+1andl=−<#comment/>1modes show, respectively, that (i) intra-modal-group crosstalk is reduced by><#comment/>5.8dBand><#comment/>5.6dBand (ii) near-error-free bit-error-rate performance is achieved with a penalty of∼<#comment/>0.6dBand∼<#comment/>3.8dB, respectively.

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  4. We experimentally demonstrate simultaneous turbulence mitigation and channel demultiplexing in a 200 Gbit/s orbital-angular-momentum (OAM) multiplexed link by adaptive wavefront shaping and diffusing (WSD) the light beams. Different realizations of two emulated turbulence strengths (the Fried parameterr0=0.4,1.0mm) are mitigated. The experimental results show the following. (1) Crosstalk between OAMl=+1andl=−<#comment/>1modes can be reduced by><#comment/>10.0and><#comment/>5.8dB, respectively, under the weaker turbulence (r0=1.0mm); crosstalk is further improved by><#comment/>17.7and><#comment/>19.4dB, respectively, under most realizations in the stronger turbulence (r0=0.4mm). (2) The optical signal-to-noise ratio penalties for the bit error rate performance are measured to be∼<#comment/>0.7and∼<#comment/>1.6dBunder weaker turbulence, while measured to be∼<#comment/>3.2and∼<#comment/>1.8dBunder stronger turbulence for OAMl=+1andl=−<#comment/>1mode, respectively.